The present invention relates in general to measurement of ground surface movements and land surface subsidence.
River floodplains are some of the most dynamic surfaces of our landscape. In the normal course of a year, the sediment surface can be flooded by several meters of water for months and then crack when the water recedes and the sediment dries. Over time, deposited sediments decompose, dewater, and compact resulting in subsidence that is sometimes offset by subsequent deposition. In situations where deposition does not occur, net elevation loss results. Historically, shallow subsidence (<30 m) has been measured with Surface Elevation Tables (SETs) 100 (Cahoon et. al, 1999), such as the manual rod SET (rSET) illustrated in FIG. 1. The distance between a reader arm 102 and a land surface 104 is measured repeatedly over time and is compared with deposition in the area to determine subsidence rates and net elevation loss or gain.
The inventor has used rSETs in riparian environments and has observed large (greater than 10 mm) surface elevation variations during rSET measurement. The observed variations were assumed to have resulted from rainfall and changes in river stage, but periodic measurements from the rSETs were inadequate to fully understand what had occurred (Kroes and Hupp 2010). As illustrated in FIG. 2, in most cases, polyvinyl chloride (PVC) sleeves 202 and marker pipes 204 associated with the SETs and other sediment monitoring locations showed dark, organic staining 206 of the upper sections and pure white lower sections 208. In one observation, the distance from the sleeve 202 at ground level 210 to the top of the white portion 208 was about 100 mm. These marks were assumed by the inventor to be from the unconsolidated sediment's shrink and swell, that is, from sediment swelling during high water that covered the sleeve 202 and prevented organic rich water from coming into direct contact. Strict conformity of environmental conditions were observed in order to minimize these variations. Close attention was paid to river stage, rainfall, and the time of year. The SETs were read during low flow, no rain for a week, and just before the leaves fell in autumn, and still had unexplained inconsistencies in surface levels. The inventor has observed that traditional rSETs located in the Atchafalaya River Basin in Louisiana have measured rates of subsidence ranging from 2.3 mm/yr to 25 mm/yr and deposition ranging from 6 to 14 mm/yr during 2010-2012.
Measurement of shallow subsidence in a river floodplain has been problematic with relatively large measurement errors if a very specific set of conditions were not closely followed. Often years of data were unusable due to what appeared to be a change in the hydration state of the floodplain. The large range in water levels on a floodplain and the typical lack of usable well casings made dry land extensiometers useless. Thus, an improvement is needed in measuring shallow land surface subsidence.